This invention relates to a process for preparing a semiconductor device, particularly to a silicon bipolar transistor with the base and the emitter being grown by a molecular beam epitaxial growth apparatus and a composite device thereof.
The process for preparing a silicon bipolar transistor according to the prior art is described by referring to FIGS. 1 (a) to (f).
First, as shown in FIG. 1 (a), on a P-type silicon substrate 11 is grown an N.sup.- -type epitaxial layer 13 with a thickness of 1.3 to 1.6 .mu.m, a specific resistivity of 1.3 to 1.0 .OMEGA..cm, and a silicon oxide film 6 with a thickness of 500 to 600 .ANG. is formed according to the spot LOCOS method.
Next, as shown in FIG. 1 (b), phosphorus is ion injected with a photoresist as the mask to form an N.sup.+ -type collector withdrawal portion 23, followed by ion injection of boron with a photoresist as the mask to form a base 14.
Next, as shown in FIG. 1 (c), a silicon nitride film 7 with a thickness of 1000 to 1500 .ANG. is deposited according to the CVD method, and then the silicon oxide film 6 of the emitter and collector contact, and the silicon nitride film 7 of the emitter, collector contact and base contact are etched according to the RIE method by use of a gas such as CF.sub.4, etc. Thereafter, a polysilicon 17 with a thickness of 1500 .ANG. is grown according to the CVD method, and arsenic is ion injected to form an N.sup.+ type emitter 16.
Next, as shown in FIG. 1 (d), with a photoresist as the mask, unnecessary polysilicon is removed by dry etching by use of a gas such as CF.sub.4, etc.
Next, as shown in FIG. 1 (e), with a photoresist as the mask, the silicon oxide film 6 at the base contact portion is etched to have boron thermally diffused, thereby forming a p.sup.+ -type external base 15.
Next, as shown in FIG. 1 (f), by forming the base electrode 9, the emitter electrode 10 and the collector electrode 22, the device portion is completed.
Thus, for the emitter of the silicon bipolar transistor, polysilicon growth and the arsenic ion injection method or the DOPOS method are employed.
Then, by high temperature heat treatment, activation of arsenic and emitter junction formation are effected.
On the other hand, MBE apparatus has begun to be applied to formation of a thin base layer of silicon bipolar transistor as the low temperature growth means in which a steep impurity distribution or a silicon-germanium mixed crystal (hereinafter abbreviated as SiGe mixed crystal) is obtained.
Even if a thin base may be grown elaborately by means of MBE apparatus, when high temperature heat treatment is applied thereafter for activation of arsenic in the emitter, the impurity profile will be changed to deteriorate the characteristics.
Even by use of an SiGe mixed crystal without crystal defect grown by MBE apparatus, when heat treatment is thereafter effected at the growth temperature or higher, misfit or dislocation may be generated, whereby there has been involved the problem that the leak current is increased, etc.
Even if it is attempted to form an emitter at low temperature by means of MBE apparatus, antimony which has been generally employed as the N-type impurity source of MBE is low in solid solubility, whereby there has been also involved the problem that no high concentration doping necessary for emitter can be effected.